This invention relates generally to hard disk drive control and, more particularly, to suppressing the influence of external mechanical vibrations on a hard disk drive system.
Hard disk drives (HDDs) are used in various environments, and sometimes they suffer performance degradation due to external mechanical vibrations. In the past, when a disk drive had a low data capacity, the track pitch was larger and the mechanical vibration reflected less head motion compared to the data track pitch. Recently, however, the track pitch has become very narrow and even small vibrations can cause large off-track motions, yielding lower performance. Previously, such degradation due to external vibrations was encountered only in unusual operating environments, but today it can be observed as performance degradation under typical operating conditions. While vibration due to disk rotation has a narrow band, external mechanical vibration can have a substantially wider band. A number of techniques may be used to address the problem caused by external vibrations.
The acceleration feedforward control method utilizes the rotational vibration sensor output and feeds it forward to the actuator control signal so that the head motion counteracts the vibrations which move the head off-track. This method works effectively when the vibration is rotational, for which the gain and the phase of the transfer function from the sensor output and the position error signal is known and does not change very much. However, the head position is not only affected by the in-plane rotational vibration, but can also be affected by various kinds of linear and rotational vibrations which cannot be picked up by the rotational vibration sensors.
The chasing peak filter method uses a peak filter that changes its frequency adaptively and tracks the frequency changes of the vibration source. The peak filter works to increase the error rejection capability at a certain frequency. Because the peak filter has a narrow bandwidth, it does not work effectively for the case where the vibration has multiple peak frequencies or a wider spectrum.
A method has been used to identify the best peak filter frequency at the beginning of the drive power-on process by scanning over peak frequencies. This method only works for the case where the vibration characteristics do not change over time. Even when this method can be used for the data recovery process, the performance drawback will still be quite large since the method takes a rather long time.
The shock mount is a HDD mounting mechanism for lowering the transfer functions from the external vibration source to the actual vibration of the drive, and works effectively when the vibration frequency is higher than the resonant frequency of the mounting mechanism. The problem is that the mounting mechanism tends to increase the vibration at its own resonant frequency.